With the increasing age and survival of males in the western world and the early diagnosis of prostate cancer, due to the availability of screening (such as PSA screening), prostate cancer has become one of the most commonly diagnosed tumours in the western world. More recently, there has been a large swing away from radical surgical prostatectomy, and a growing preference for treatment by brachytherapy.
Brachytherapy involves the permanent implantation of a plurality of radioactive seeds (each comprising an X-ray source) into the patient's prostate. Ir-192, I-125 and Pd-103 sources are commonly employed. The seeds are implanted one at a time by means of a dedicated type of syringe, and located within the prostrate in a predetermined pattern designed to ensure both that the seeds irradiate the appropriate volume of the prostrate, and do not irradiate (or minimally irradiate) healthy tissue, most especially outside the prostrate. A template is placed against the patient's body with apertures for the syringe, which is inserted through each aperture in turn and, at each of a series of predetermined depths, a seed is released. The procedure is monitored by means of an ultrasound probe located in the rectum, so that the operator can correctly locate the seeds.
However, this existing monitoring technique is highly subjective, and can lead to incorrect dosing of various tissues by as much as a factor of two, and to the excessive dosing of the patient's urethra and rectum. These kinds of complications are very real for treatment of prostate cancer with permanent implants of I-125 or Pd-103 seeds or high dose brachytherapy (HDB) by Ir-192 sources.
The prostate low dose brachytherapy procedure for early stage disease involves the permanent implantation of radioactive seeds into the prostate, normally in the form of I-125 and Pd-103 seeds. Both of these seeds are gamma ray emitters: I-125 (Eγ˜27 keV, T1/2=60 days, initial dose rate 8 cGy/h), Pd-103 (Eγ˜21 keV, T1/2˜17 days, initial dose rate 20 cGy/h). I-125 and Pd-103 implanting, in comparison with other competing treatment modalities such as X-rays from a LINAC, delivers a much higher dose to the target than could safely be administered by an external beam of radiation. Another advantage of using I-125 and Pd-103 seeds is the short tissue penetration of the gamma photons due to the low photon energy of the radiation (half layer is 1.3 cm for I-125 and even less for Pd-103).
Another treatment method, for more advance disease, is high dose rate brachytherapy utilising insertion of a high activity (10 Ci, 400 GBq) Ir-192 source for three to four short fractions.
However, even an ideal pre-implant plan of dose distribution does not guarantee a well delivered dose as may be demonstrated in a post implant evaluation. Misplacement of seeds can often lead to severe complications such as impotence and urinary incontinency, which sometimes arises due to overdosing of the neuro-vascular bundle and urethra.
A clear need exists, therefore, for improved techniques for prostate brachytherapy that allow quality assurance in real time. For interstitial brachytherapy the achievements of local control for prostate cancer is greatly influenced by the dose distribution generated by implanted radionuclide seeds. The treatment plan must be able to deliver the prescribed dose in a tumour, with adequate margins, while minimizing the dose delivered to the surrounding healthy tissues. A sophisticated dose planning procedure for interstitial brachytherapy demands a knowledge of dose distribution around the low dose rate and low X-ray energy radioactive seeds, in the case of I-125 and Pd-103 and high dose rate gamma sources in case of Ir-192. Existing commercial hospital treatment planning systems nevertheless still employ traditional dose calculation formulae in their interstitial brachytherapy source calculation algorithms.
It is an object of the present invention, therefore, to provide an improved dosimetry method and apparatus, which can be used for monitoring radiation dose or source location in a one or more source environment, and which—in one embodiment—can be used to control dose.